empty_struct_with_brackets: do not lint macro code

Do not attempt to fetch a snippet from expansion. Without this change,
the inside of macros could[*] be shown as the source of the problem.

[*] Due to the way the source code is processed and reparsed in this
macro, the declarative macro has to be located outside the current
source file for the bug to appear. Otherwise, the macro call itself
will be (mis)identified as a potential `struct` field definition and the
lint will not trigger.

Author: samueltardieu

clippy_lints/src/empty_with_brackets.rs

@@ -75,6 +75,7 @@ declare_lint_pass!(EmptyWithBrackets => [EMPTY_STRUCTS_WITH_BRACKETS, EMPTY_ENUM
 impl EarlyLintPass for EmptyWithBrackets {
     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
         if let ItemKind::Struct(ident, var_data, _) = &item.kind
+            && !item.span.from_expansion()
             && has_brackets(var_data)
             && let span_after_ident = item.span.with_lo(ident.span.hi())
             && has_no_fields(cx, var_data, span_after_ident)

tests/ui/auxiliary/macro_rules.rs

@@ -57,3 +57,10 @@ macro_rules! bad_transmute {
         std::mem::transmute($e)
     };
 }
+
+#[macro_export]
+macro_rules! empty_struct {
+    ($i:ident) => {
+        struct S {}
+    };
+}

tests/ui/empty_structs_with_brackets.fixed

@@ -1,3 +1,4 @@
+//@aux-build:macro_rules.rs
 #![warn(clippy::empty_structs_with_brackets)]
 #![allow(dead_code)]
 
@@ -23,4 +24,6 @@ struct MyTupleStruct(usize, String); // should not trigger lint
 struct MySingleTupleStruct(usize); // should not trigger lint
 struct MyUnitLikeStruct; // should not trigger lint
 
+macro_rules::empty_struct!(FromMacro);
+
 fn main() {}

tests/ui/empty_structs_with_brackets.rs

@@ -1,3 +1,4 @@
+//@aux-build:macro_rules.rs
 #![warn(clippy::empty_structs_with_brackets)]
 #![allow(dead_code)]
 
@@ -23,4 +24,6 @@ struct MyTupleStruct(usize, String); // should not trigger lint
 struct MySingleTupleStruct(usize); // should not trigger lint
 struct MyUnitLikeStruct; // should not trigger lint
 
+macro_rules::empty_struct!(FromMacro);
+
 fn main() {}

tests/ui/empty_structs_with_brackets.stderr

@@ -1,5 +1,5 @@
 error: found empty brackets on struct declaration
-  --> tests/ui/empty_structs_with_brackets.rs:4:25
+  --> tests/ui/empty_structs_with_brackets.rs:5:25
    |
 LL | pub struct MyEmptyStruct {} // should trigger lint
    |                         ^^^
@@ -9,7 +9,7 @@ LL | pub struct MyEmptyStruct {} // should trigger lint
    = help: remove the brackets
 
 error: found empty brackets on struct declaration
-  --> tests/ui/empty_structs_with_brackets.rs:6:26
+  --> tests/ui/empty_structs_with_brackets.rs:7:26
    |
 LL | struct MyEmptyTupleStruct(); // should trigger lint
    |                          ^^^